Method and apparatus for aligning a vehicle with an inductive charging system

ABSTRACT

A method and apparatus for aligning a vehicle with an inductive charging system is characterized by the addition of alignment coils to a secondary coil on the vehicle. For efficient inductive charging, it is necessary that the vehicle mounted secondary coil be aligned with a stationary primary coil of a transformer of the inductive charging system. When the primary coil is energized, it produces a magnetic field which induces a voltage in the alignment coils as a function of the proximity of the alignment coils to the central axis of the primary coil. The voltage differential between opposed pairs of alignment coils is determined by a comparator which then generates a directional signal which can be used by the operator of the vehicle to position the vehicle for closer alignment of the vehicle secondary coil with the primary coil and more efficient charging.

This application is a continuation-in-part of U.S. application Ser. No.13/198,863 filed Aug. 5, 2011.

BACKGROUND OF THE INVENTION

Electric vehicle energy storage systems are normally recharged usingdirect contact conductors between an alternating current (AC) sourcesuch as is found in most homes in the form of electrical outlets;nominally 120 or 240 VAC. A well known example of a direct contactconductor is a two or three pronged plug normally found with anyelectrical device. Manually plugging a two or three pronged plug from acharging device to the electric automobile requires that conductorscarrying potentially lethal voltages be handled. In addition, theconductors may be exposed, tampered with, or damaged, or otherwisepresent hazards to the operator or other naïve subjects in the vicinityof the charging vehicle. Although most household current is about 120VAC single phase, in order to recharge electric vehicle batteries in areasonable amount of time (two-four hours), it is anticipated that aconnection to a 240 VAC source would be required because of the size andcapacity of such batteries. Household current from a 240 VAC source isused in most electric clothes dryers and clothes washing machines. Theowner/user of the electric vehicle would then be required to manuallyinteract with the higher voltage three pronged plug and connect it atthe beginning of the charging cycle, and disconnect it at the end of thecharging cycle. The connection and disconnection of three pronged plugscarrying 240 VAC presents an inconvenient and potentially hazardousmethod of vehicle interface, particularly in inclement weather.

In order to alleviate the problem of using two or three prongedconductors, inductive charging systems have been developed in order totransfer power to the electric vehicle. Inductive charging, as is knownto those of skill in the art, utilizes a transformer having primary andsecondary windings to charge the battery of the vehicle. The primarywinding is mounted in a stationary charging unit where the vehicle isstored and the secondary winding is mounted on the vehicle

To maximize efficiency, it is important that the secondary winding onthe vehicle be aligned with the primary winding in the stationarycharging unit. The present invention relates to inductive proximitycharging. More particularly, the invention relates to a system and forassisting the operator with positioning the vehicle so that thesecondary winding thereon is in close proximity and aligned with thestationary primary winding for efficient inductive charging of thevehicle.

BRIEF DESCRIPTION OF THE PRIOR ART

Inductive charging systems are well known in the prior art. For example,the Partovi U.S. patent application publication No. 2009/0096413discloses an inductive charging system in which includes a base unitcontaining a primary coil and mobile device including a secondarywinding. To assist with alignment of the mobile device and the baseunit, a plurality of alignment magnets are provided behind each coil.The magnets behind the primary and secondary coils are arranged inpairs, respectively, with the poles of each pair being opposite so thatthe magnets will attract and thus align the coils.

While the prior devices operate satisfactorily, they are not suitablefor use in vehicle charging systems. First, the magnets add unnecessaryweight to the vehicle coil which decreases the efficiency of thevehicle. Second, the magnets are not strong enough to reposition thevehicle or base unit relative to one another.

The present invention was developed in order to overcome these and otherdrawbacks of the prior alignment techniques by providing an alignmentapparatus which assists the operator of a vehicle in positioning thevehicle so that the secondary coil mounted thereon is in close proximityto and aligned with the stationary primary coil in the base unit formaximum inductive energy transfer to a battery charger on the vehicle.The operator of the vehicle is any person or thing that has the abilityto control the location of the vehicle. Examples of vehicle operatorsare human drivers, robots, and computer systems.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the invention to provide a methodand apparatus for aligning a vehicle with an inductive charging system.The apparatus includes a transformer having a stationary primary coiland a secondary coil mounted on the vehicle. A plurality of alignmentcoils are spaced in opposing pairs across an axis of symmetry in thevicinity of the secondary coil. When the primary coil is energized, itgenerates a time harmonic, or alternating current magnetic field whichinduces voltage in the alignment coils as a function of the proximity ofeach alignment coil to the central axis of the primary coil. Thealignment coils are connected with a controller which generates anoutput signal corresponding to the position of the vehicle relative tothe primary coil. The output signal is utilized to communicateinformation, via a display or the like, which is used by the operator ofthe vehicle to position the vehicle so that the secondary coil andprimary coils are axially aligned.

The alignment coils each have the same diameter which is significantlyless than the diameter of the secondary coil. The alignment coils arearranged symmetrically within the vicinity of the secondary coil and mayhave axes which are parallel to the axis of the secondary coil.Alternatively, for three-dimensional coils, the orientation of the axisbetween the alignment coils and the primary and secondary coils isarbitrary.

A voltage sensor is connected with each alignment coil for measuring thevoltage induced in each alignment coil by the primary coil. The voltagesensors are connected with a comparator within the controller. Thecomparator measures the voltage differential between the voltage sensorsto generate a directional signal relative to the axis of the secondarycoil. The directional signal contains information which is communicatedeither wirelessly to an external display or directly to a display withinthe vehicle to provide a visual indication to the vehicle operator ofthe direction that the vehicle must be moved in order to bring thevehicle secondary coil into alignment with the stationary primary coil.

According to a further object of the invention, the alignment coils canbe arranged as an assembly of coils to measure magnetic field componentsalong different spatial axes in order to improve the accuracy indetermine the relative proximity to a magnetic field source coil.

It is yet another object of the invention to provide a plurality ofalignment coils which are positioned in such a way as to shape a targetzone for the primary coil in order to improve the alignment of thesecondary coil with respect to the primary coil.

BRIEF DESCRIPTION OF THE FIGURES

Other objects and advantages of the present invention will becomeapparent from a study of the following specification when read inconjunction with the accompanying drawing, in which:

FIG. 1 is a schematic diagram of an inductive vehicle charging systemincorporating an alignment system according to the invention;

FIGS. 2 and 3 are top and side views, respectively, of the primary,secondary and alignment coils of the alignment system of FIG. 1;

FIG. 4 is a schematic diagram of the secondary and alignment coils andcontroller of the alignment system of FIG. 1;

FIGS. 5 and 6 are top and side views, respectively, of source anddetector coils of a proximity detector;

FIG. 7 is graph showing the induced voltage along a plane of thedetector coils shown in FIGS. 5 and 6;

FIGS. 8 and 9 are top and side views, respectively, of source anddetector coils of a proximity detector with the detector coils indifferent positions than those shown in FIGS. 5 and 6;

FIG. 10 is a graph showing the induced voltage along a plane of thedetector coils shown in FIGS. 8 and 9;

FIG. 11 a perspective view of a multi-axis coil assembly according toanother embodiment of the invention;

FIG. 11A is a schematic diagram of a plurality of alignment coilassemblies spaced in the vicinity of a secondary coil.

FIG. 12 is a schematic diagram of circuitry used to calculate themagnetic flux magnitude of the multi-axis coil assembly of FIG. 11;

FIG. 13 is a graph showing the induced voltage using a multi-axis sensorin the positions shown in FIGS. 8 and 9;

FIG. 14 is a schematice diagram of alternate circuitry used toapproximate the magnetic field magnitude of the multi-axis coil assemblyof FIG. 11;

FIG. 15 is a schematic representation of a target zone for a proximitydetection system using three coils;

FIG. 16 is a schematic representation of a target zone for a proximitydetection system using four coils;

FIG. 17 is a diagram showing the preferred embodiment of alignmentzones;

FIG. 18 is a diagram showing the preferred embodiment of sub-alignmentzones; and

FIG. 19 is a diagram showing an alternate configuration of sub-alignmentzones.

DETAILED DESCRIPTION

Referring first to FIG. 1, there is shown an inductive charging systemfor electric vehicles. The system includes a charging station 2 and atransformer 4. The transformer includes a stationary primary coil 6which is preferably mounted on the ground such as the floor of a garage.The primary coil is connected with the charging station. The transformerfurther includes a secondary coil 8 which is mounted on a vehicle 10.The secondary coil is mounted at a location on the vehicle so that thevehicle can be positioned adjacent to the charging station with thesecondary coil above the primary coil as shown. Preferably, the coilsare arranged with their axes in alignment for maximum energy transferthere between. The charging station 2 is connected with a power source12.

The system for positioning the vehicle to align the vehicle secondarycoil 8 with the stationary primary coil 6 will be described withreference to FIGS. 2-4. Referring first to FIGS. 2 and 3, the primarycoil 6 when energized produces a magnetic field 10. The secondary coil 8includes a plurality of alignment coils 14. In the embodiment shown,four alignment coils 14 a-d are provided, although it will beappreciated by those of ordinary skill in the art that greater or lessthan four alignment coils may be provided as will be developed below.

The alignment coils are symmetrically arranged in the vicinity of thesecondary coil. In a preferred embodiment shown in FIG. 2, the alignmentcoils 14 are spaced in opposing pairs across an axis of symmetry withinthe inner circumference of the secondary coil 8. The alignment coils arepreferably of the same size and configuration and have diameterssignificantly less than the diameter of the secondary coil. They arepreferably arranged adjacent to the inner circumference of the secondarycoil and may have axes which are parallel to the axis of the secondarycoil as shown in FIG. 3. When the primary coil is energized by thecharging station 2, voltages are induced in the alignment coils inaccordance with the proximity of the alignment coils to the magneticfield 10. Alternatively, for three-dimensional coils, the orientation ofthe axis between the alignment coils and the primary and secondary coilsis arbitrary. For a one-dimensional coil, the alignment of the axes ofthe alignment coils is more complicated than just being arrangedparallel to the axis of the secondary coil. The opposing pairs ofalignment coils are preferably arranged to capture an equal amount offlux from the primary coil when the primary and secondary coils areproperly aligned.

In an alternate embodiment, the alignment coils may be arrangedexternally of the secondary coil. In either embodiment, the alignmentcoils are spaced from each other in opposing pairs in a symmetricalarrangement relative to the secondary coil as will be developed ingreater detail below.

Referring now to FIG. 4, the alignment system according to the inventionfurther includes a controller 16 connected with the secondary coil 8. Aswill be developed below, the controller generates an output directionalsignal corresponding to the position of the vehicle secondary coilrelative to the primary coil. A display 18 is connected with thecontroller to provide a visual indication of the output signal which isused by the operator of the vehicle to position the vehicle and axiallyalign the secondary coil with the primary coil for maximum inductionenergy transfer to the secondary coil to power a battery charger (notshown) on the vehicle. The controller 16 includes plurality of voltagesensors 20 connected with the alignment coils 14. Thus in the embodimentshown, voltage sensors 20 a-d are connected with alignment coils 14 a-d,respectively. The voltage sensors measure the voltage induced in eachcoil by the primary coil. The controller also includes a comparator 22connected with the voltage sensors 20. The comparator measures thevoltage differential between the voltage sensors to generate thedirectional signal.

Preferably, an even number of alignment coils are provided, with thecoils on opposite sides of the secondary coil being paired. Thus, in theembodiment shown in FIGS. 2-4, coils 14 a and 14 c are paired and coils14 b and 14 d are paired. Referring to FIG. 3, the voltage induced incoil 14 a is greater than the voltage induced in coil 14 c because coil14 a is closer to the magnetic field. The comparator 22 measures thevoltage differential between the opposed pairs of coils and generates adirectional signal for each of the opposed pairs. The directionalsignals from the opposed pairs of coils are combined by the comparatorto generate a composite directional signal which is displayed on thedisplay 18 and used by the operator of the vehicle to position thevehicle secondary coil proximate to the primary coil.

As noted above, the charging station 2 is connected with a power source12. The power source is preferably a 220 volt AC supply operating atbetween 50 and 60 Hz. For operation of the alignment system according tothe invention, the primary coil 6 is initially energized by the powersource and charging station at a reduced voltage so that the magneticfield 10 is produced which is symmetric about the axis of the primarycoil. The alignment coils are arranged in the secondary coil so that themagnetic flux produced by the stationary primary coil 6 induces avoltage in each alignment coil which is proportional to its proximity tothe center of the primary coil. The voltage sensors 20 measure therelative amount of voltage induced in each alignment coil, and thecomparator determines the direction of misalignment based on the voltagedifferential in the opposed pairs of coils. For example, if the vehiclemounted secondary coil 8 is positioned relative to the stationaryprimary coil 6 as shown in FIGS. 2 and 3, the voltage induced inalignment coil 14 a will be larger than in coil 14 c. This informationcan be used to indicate that the vehicle needs to be moved in thedirection of coil 14 a in order to improve the alignment. When thevoltage in all four alignment coils 14 a-d is equal, the vehiclesecondary coil 8 will precisely aligned with the stationary primarycoil. Once aligned, the power supplied by the power source to theprimary coil is increased to begin the charging process. Moreparticularly, the charging station includes a power converter whichconverts the incoming source voltage from the power supply into asinusoidal voltage of arbitrary frequency and voltage. The sinusoidalvoltage is supplied to the stationary primary coil 6. Current within theprimary coil generates a magnetic field which induces a current in thesecondary coil 8 mounted on the vehicle. This in turn produces an outputvoltage which is delivered to a battery charger (not shown) in thevehicle to charge the vehicle battery.

It will be appreciated that the alignment system according to theinvention may be provided with only a single pair of opposed alignmentcoils if only two-direction misalignment information is desired.Additional pairs of alignment coils may be provided for more precisealignment information.

The inductive charging system may complement a conventional conductivecharger. The controller 16 is operable to control both types of chargingas well as operation of the alignment system. A transfer switch (notshown) on the controller is operable to isolate the charging sources toprevent the user from simultaneously using both inductive and conductionrecharging systems.

FIGS. 5 and 6 illustrate a magnetic field source coil 102 and a detectorcoil 104 of a promity detection system. The detector coil 104 is shownin two positions, one in phatom, relative to the coil 102. As showntherein, the measurement or detector coil is located in close proximityto the source coil which is creating the magnetic field. In this case,the axis of the detector coil is significantly different than thedirection of the magnetic flux, and the voltage induced in the detectorcoil accurately reflects the magnitude of the magnetic flux through thecoils. However, the flux lines from the primary coil do not have auniform direction throughout the plane within which the alignment coilmoves. Accordingly, the alignment coil at the two different locationswithin this plane shown in FIGS. 5 and 6 each capture flux lines at adifferent angle leading to an induced voltage that does not necessarilyincrease as the alignment coils are moved closer to the primary coil. Infact, with single axis alignment coils as shown, there will always be adead zone where the induced voltage falls off and then rises back up asthe coils become tangent to the field lines. For a single axis coil thatis parallel to the primary or charging coil, this occurs when thedetector coil 104 is placed directly above the edge of the primary coil.The induced voltage for the detector coil in the positions shown inFIGS. 5 and 6 is shown in FIG. 7.

FIGS. 8 and 9 show additional positions of the detector coil whichcannot be distinguished by the sensors. The induced voltage for thedetector coil in the positions shown in FIGS. 8 and 9 is shown in FIG.10.

According to another embodiment of the invention, a multi-coil assemblyis used in place of single coils to produce more accurate proximitymeasurements based on magnetic flux measurement. The preferredembodiment of a multi-coil assembly is shown in FIG. 11 where themulti-coil assembly 202 includes three coils C_(x), C_(y), and C_(z),with each coil being oriented orthogonally to the plane defined by theaxis of the other two coils. When in the presence or vicinity of a timevarying magnetic field, the voltages V_(x), V_(y), and V_(z) induced ineach coil will be proportional to the magnitude of the spatial componentof the magnetic field in the direction of each coil axis. Because themagnetic flux density magnitude |B_(t)| can be expressed as a functionof the x, y, and z spatial components B_(x), B_(y), and B_(z) as|B _(t)|=√(|B _(x)|² +|B _(y)|² +|B _(z)|²)  equation 1

-   a value proportional to |B_(t)| can be calculated based on the    measured coil voltages V_(x), V_(y), and V_(z), so that |B_(t)| is    proportional to √(|V_(x)|²+|V_(y)|²+|V_(z)|²). This calculation can    be implemented using external circuitry and any of a variety of well    understood techniques which may be analog or digital. An example of    a circuit for calculating the magnetic flux density magnitude    |B_(t)| for the coils C_(x), C_(y), and C_(z) is shown in FIG. 12    using a processor 204 connected with the coils for processing the    voltages of each.

However, for the purpose of determining the relative location of twoalignment coil assemblies, this calculation can be simplified. Forexample, the quantity V_(sum) _(_) _(abs), defined by the equationV _(sum) _(_) _(abs)=(|V _(x) |V _(y) |V _(z)|)  equation 2

-   can be used to determine the relative position of the coil    assemblies with respect to a primary coil which is creating the    magnetic field. This calculation requires less complex external    circuitry to process the coil voltage measurements. For example, the    external circuitry may include simple passive components such as a    diode rectifier and low pass filter applied to each coil. In FIG.    13, the individual rectified coil outputs are connected in series in    order to approximate the magnetic field magnitude. The coils are    connected in series with diodes 302 and resistors 304 and in    parallel with capacitors 306. The voltage V_(sum) _(_) _(abs) is    provided at the output of the circuit of FIG. 14.

The voltage induced in a multi-axis sensor in the positions shown inFIGS. 8 and 9 is represented by the plot shown in FIG. 13.

The emission of electromagnetic fields (EMF) is a major concern ofstandards and regulatory bodies. Because EMF drops off rapidly withdistance, one way to reduce exposure is to ensure that the primary coilis positioned well underneath the vehicle. By using multiple detectorcoils, the shape of the target zone can be biased so that the primarycoil is always positioned relative to the secondary coil in such a waythat the stray EMF emitted from underneath the vehicle is belowallowable standards or regulations. This is done by changing theacceptable threshold for V_(x) and V_(y). Thus, the size and shape ofthe target zone can be modified. FIGS. 15 and 16 show varied targetzones. In FIG. 15, the shape of the zone is defined as two conjoinedsemicircles. Optimal alignment occurs within the range of the largersemicircle shown in FIG. 15. In the event that the driver of a vehicleto be charged overshoots the stationary primary coil when parking thevehicle, the alignment zone is greatly reduced as shown by the lowersemicircle in FIG. 15 to ensure that EMF levels emitted from the vehicleremain within specified guidelines.

In FIG. 16, a fourth detector coil C₃₀₄ is added and the coils arepositioned on the desired axes. This arrangement allows for betterresolution of the center point of the target zone and the overall zonetakes on an arbitrary shape. The exact size and shape of the zone isdetermined primarily by the allowable thresholds set in the processingcircuit. The arbitrary shape for the alignment zone is preferred sinceit does not create a discontinuity at any point in the alignment zonesuch as the one that occurs between the top and bottom semicircles shownin FIG. 15. The size and shape of the alignment zone can be changed andrefined through control of the analysis of data and feedback to theuser. Additional detector coils improve the accuracy of positioning thevehicle relative to the primary coil. A fixed controller algorithm canbe used to control the shape of the alignment zone to some extent withthe position of the detector coils. For example, if the controller onlyseeks to have the detector coils all be equal, then detector coils canbe positioned around the desired center point of alignment even if thatis not the center point of the secondary coil. Alternatively, thecontroller can seek the desired difference in the values from thedetector coils instead of trying to make them equal in order to achievethe same effect.

The alignment zones can be used to control field levels outside theperimeter of the vehicle, but they have other uses. The alignment systemutilizes multiple zones as shown in FIGS. 17-19. Zone 1 is the zone inwhich the system will indicate to the user to stop. The second, slightlylarger Zone 2 is the area where the system will continue to indicate tostop. Zone 3 is an even larger zone which is used to define the area inwhich the charger is capable of operating. Zone 4 is beyond Zone 3 andprovides an area where guidance to move the vehicle is provided, but thecharger is not capable of operating. In operation, the alignment systemindicates to the vehicle operator to move forward, back, left, or rightuntil the vehicle enters the first zone, at which point it indicates tostop. The alignment system continues to indicate to stop as long as thevehicle is in the second zone. If the vehicle exits the second zone thenthe system again indicates to the vehicle operator to move forward,back, left, or right until the vehicle enters the first zone, at whichpoint it indicates to stop. When the vehicle is turned off, thealignment system verifies that it is still within the third zone beforebeginning charging.

The first and second zones provide a form of hysteresis around theresponse time of the driver to the feedback from the system. This isespecially important when the driver just grazes the edge of first zoneand is only in this area for a fraction of an inch. The second and thirdzones are needed because of the amount that the vehicle can roll or rockafter being turned off. This is particularly a problem if the parkingbrake is not set. By forcing the driver to align slightly closer thanneeded to charge, car shifting after the car has been parked and thedriver has exited the vehicle can still be accommodated.

Various types of feedback to the user during positioning of the vehiclemay be used. By way of example only, a display of scrolling arrows thatslow down as the vehicle gets closer to the primary coil may beprovided. The speed of the arrows is proportional to distance from thevehicle to the primary coil. When the vehicle needs to be moved left,right, or backwards, these indicators are illuminated and flash at afixed rate. Once the user is in position, a green ring is illuminated toindicate that the user should stop.

While the preferred forms and embodiments of the present invention havebeen illustrated and described, it will be readily apparent to thoseskilled in the art that various changes and modifications may be madewithout deviating from the inventive concepts set forth above.

What is claimed is:
 1. Apparatus for aligning a vehicle with an inductive charging system, comprising: (a) a transformer including a stationary primary coil and a secondary coil mounted on the vehicle; (b) at least one alignment coil assembly including at least three assembly coils, each assembly coil having a longitudinal axis and oriented orthogonally to a plane defined by the longitudinal axes of two other assembly coils, said alignment coil assembly being arranged in the vicinity of said secondary coil, said primary coil generating a magnetic field when energized to induce a voltage in each assembly coil; and (c) a controller connected with said alignment coil assembly for generating an output signal corresponding to the voltage induced in each assembly coil by the magnetic field, each voltage corresponding to the position of the secondary coil relative to the primary coil.
 2. Apparatus as defined in claim 1, wherein said controller includes a processor for processing the induced voltages from said assembly coils to produce said output signal which corresponds to the magnetic field through each assembly.
 3. Apparatus as defined in claim 1, wherein said controller includes a diode rectifier and low pass filter connected with each assembly coil for measuring the voltage induced therein by the magnetic field.
 4. Apparatus as defined in claim 3, wherein said diode rectifier and low pass filter for each assembly coil are connected in series to produce an output which corresponds to the magnetic field amplitude.
 5. Apparatus as defined in claim 1, wherein a plurality of alignment coil assemblies are provided, said assemblies being equally spaced in the vicinity of said secondary coil.
 6. A method for aligning a vehicle with an inductive charging system including a stationary primary coil and a secondary coil mounted on the vehicle, comprising the steps of (a) providing at least three assembly coils, each assembly coil having a longitudinal axis and oriented orthogonally to a plane defined by the longitudinal axes of two other assembly coils; (b) energizing the primary coil to generate a magnetic field which induces a voltage in each alignment coil; and (c) generating an output signal corresponding to the voltages induced in each assembly coil, each voltage corresponding to the position of the secondary coil relative to the primary coil.
 7. A method as defined in claim 6, wherein said output signal corresponds to the magnetic field amplitude. 